1. Field of the Invention
The present invention relates to a method of preparing an annular skin tube which covers an annular foam mass having an opening formed therethrough.
2. Discussion of the Prior Art
A head rest for a seat on a motor vehicle, which has a central opening, is one example of an annular foam product. The head rest is usually produced by injecting a foamable composition into an annular skin tube or cover in which a suitable core structure is set within the skin tube. A foam mass is formed by reaction of the injected foamable composition within the annular skin tube. To produce this type of head rest, the appropriate annular skin tube or cover is prepared, generally by joining a plurality of tubular skins in an end-to-end manner while the tubular skins are turned inside out, i.e., while the nominal inside of each tubular skin is exposed.
To prevent the linearly and circularly joined edges of the skin tube from being exposed or seen on the outer surface of the produced foam product, the tubular skins which have been joined together should be turned over so that the joined edges are located inside of the skin tube. If the tubular skins are completely joined together into an annular shape corresponding to the annular head rest, the obtained annular skin tube with its nominal inside exposed cannot be turned over to expose its nominal outside. To make it possible to turn over the annular skin tube, it has been practiced to provide the skin tube with an unjoined portion left on the inner periphery of the skin tube which defines the central opening. This unjoined portion of the annular skin tube permits the skin tube to be turned over to expose its nominal outside, in order that the linearly and circularly joined edges are located inside of the annular skin tube.
The unjoined portion provided in the annular skin tube is also utilized for positioning the core structure within the annular skin tube.
On the other hand, the unjoined portion left in the prepared annular skin tube should eventually be closed or joined after the skin tube is turned over to expose its nominal outside. Therefore, it is necessary that the unjoined portion be joined such that the joined edges are hidden inside of the skin tube. However, this joining procedure is very difficult and time-consuming.
In the light of the above drawback encountered in the conventional method of preparing an annular skin tube, laid-open Publication No. 62-35811 (published Feb. 16, 1987) of unexamined Japanese Patent Application proposes a technique for closing the unjoined portion left in the annular skin tube, by using gripping plates as indicated at 100, 102 in FIG. 16. Described more specifically, the gripping plates 100, 102 have longitudinally extending recess and projections, respectively. After the annular skin tube 104 with the unjoined portion is turned over to expose its nominal outside, the unjoined edges of the annular skin tube are gripped by and between the gripping plates 100, 102, such that the projection on the gripping plate 102 snaps into the recess in the gripping plate 100.
The use of the gripping plates 100, 102 considerably increases the cost of manufacture of the annular skin tube, and therefore the cost of manufacture of the foam product. Further, the gripping plates are made of a relatively hard material, which deteriorates the cushioning characteristic of the foam product, particularly where the product is a cushioning article such as a head rest for a motor vehicle. Moreover, the gripping plates should have sufficient dimensions, particularly in the direction perpendicular to their length. Consequently, the provision of the gripping plates within the annular skin tube reduces the freedom of design of the foam product.